Device and method for facilitating the focusing of a camera

ABSTRACT

A device and a method for facilitating the focusing of a camera during a so-called tilt operation, in which an image plane/focusing plane of a camera is angled relative the optical axis of an objective lens or vice versa, in order to obtain an inclined plane of focus, the camera being provided with a focusing screen located in said image plane/focusing plane and an optical viewing location, the optical axis which coincides with the optical axis of said focusing screen, said tilt operation resulting in the exiting light from the focusing screen being directed away from said viewing location, wherein an optical wedge is located between said focusing screen and said viewing location in such a way as to redirect the exiting light to said viewing location at which an eyepiece or an observer&#39;s eye is located. In a preferred embodiment, said optical wedge is a fresnel prism plate located adjacent to said focusing screen on the side of the exiting light.

TECHNICAL FIELD

The present invention relates to a device and a method for facilitatingthe focusing of a camera, more particularly for the focusing of a cameraduring a so-called tilt operation in which an image plane/focusing planeof the camera is angled relative the optical axis of an objective lensor vice versa, in order to obtain an inclined plane of focus inaccordance with the so-called Scheimpflug theorem.

BACKGROUND OF THE INVENTION

The viewfinder of a camera includes a focusing screen, on which theimage of the selected object is projected. Said focusing screen islocated in the image plane/focusing plane of the camera and theprojected image on the focusing screen may thus be used for focusingadjustment. The focusing screen normally comprises a matte surface and athin, positive fresnel lens abutting said matte surface. The fresnellens serves to redirect the light from the matte surface (and especiallyfrom its edges) towards an observer's eye or an eyepiece, through whichthe image on the focusing screen is observed. The fresnel lens thushelps to obtain an evenly distributed light-intensity across the entireimage-area.

The focusing screen is normally positioned perpendicularly to theoptical axis of the camera. In some cameras, however, it is possible toangle or "tilt" the image plane/focusing plane relative the optical axisof an objective lens or vice versa, in a so-called tilt operation inorder to obtain an inclined plane of focus. This helps the photographerto decide more easily which parts of the object that are to be in focus.

However, a serious problem arises when the incident light falls at anangle to a focusing screen comprising a fresnel lens.

The problem lies in that the exiting light has virtually the samedirection as the incident light, which means that the light is directedaway from the observer's eye or the eyepiece normally located directlyabove the center of the focusing screen. Thus, only a small fraction ofthe exiting light from the focusing screen can be viewed by thephotographer, which makes focusing very difficult or--at larger tiltangles--impossible.

If the eyepiece is moved sideways so as to catch the exiting light, thephotographer is forced to look through the eyepiece at an angle, whichmeans that the eyepiece is used in an optically very unfavourablemanner.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a deviceand a method for facilitating the focusing of a camera during a tiltoperation, in order to compensate for the drawbacks described above.

This object is achieved in accordance with the present invention bymeans of a device and a method for facilitating the focusing of a cameraduring a so-called tilt operation, in which an image plane/focusingplane of a camera is angled relative the optical axis of an objectivelens or vice versa, in order to obtain an inclined plane of focus, thecamera being provided with a focusing screen located in said imageplane/focusing plane and an optical viewing location, the optical axisof which coincides with the optical axis of said focusing screen, saidtilt operation resulting in the exiting light from the focusing screenbeing directed away from said viewing location, wherein an optical wedgeis located between said focusing screen and said viewing location insuch a way as to redirect the exiting light to said viewing location atwhich an eyepiece or an observer's eye is located.

In a preferred embodiment, said optical wedge is a fresnel prism platelocated adjacent to said focusing screen on the side of the exitinglight.

In an advantageous embodiment of the invention, a plurality of fresnelprism plates having progressively larger deflection angles, areremovably insertable adjacent to said focusing screen.

Said deflection angles preferably range from 0 to 30 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample only, and with reference to the attached drawings, in which:

FIG. 1 schematically illustrates a conventional camera during a tiltoperation, showing how the exiting light from the focusing screen isdirected away from the eyepiece;

FIG. 2 schematically illustrates a camera provided with a device inaccordance with the invention, during a tilt operation, showing how theexiting light from the focusing screen is redirected to the eyepiece bymeans of an optical wedge in the form of a fresnel prism plate;

FIG. 3 schematically shows a prior art situation in a large-formatcamera provided with a magnifying lens, during a tilt operation.

FIG. 4 schematically shows a large-format camera provided with amagnifying lens, and a fresnel prism plate in accordance with theinvention during a tilt operation.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1 a much simplified illustration of a conventional camera isshown during a tilt operation and this helps to demonstrate the problemsunderlying the present invention. FIG. 2 shows an almost identicalcamera, but provided with a device in accordance with the invention.FIGS. 1 and 2 both schematically depict the exit pupil of an objectivelens denoted by reference numeral 1, a focusing screen 2 and an opticalviewing location 3, the optical axis of which coincides with the opticalaxis of said focusing screen 2.

An observer's eye or an eyepiece 5 (as shown in the figures) is locatedin said optical viewing location 3. In a conventional manner, thefocusing screen 2 comprises a matte surface and a thin, positive fresnelprism (not shown).

In FIG. 1 it is clearly seen how the exiting light from the focusingscreen 2 is directed away from the eyepiece 5 so that very little lightreaches it. In this situation the focusing procedure is clearly madevery difficult.

In FIG. 2, however, a device in accordance with the invention isprovided, during a tilt operation. The figure shows how the lightexiting from the focusing screen is redirected to the eyepiece 5 bymeans of an optical wedge in the form of a fresnel prism plate 4,located between said focusing screen 2 and said viewing location 5. Theprism plate 4 is more particularly located adjacent to said focusingscreen 2 on the side of the exiting light. The exact distance betweenthe prism plate 4 and the focusing screen is however not critical.

In the preferred embodiment of the invention, a plurality of fresnelprism plates 4, having progressively larger deflection angles, areremovably insertable adjacent to said focusing screen.

Said deflection angles preferably range from 0 to 30 degrees. Inpractice, two prism plates 4 are used to cover said range of deflectionangles.

FIGS. 3 and 4 schematically show a similar application in a large-formatcamera provided with the exit pupil of an objective lens 1, a focusingscreen 2 and an optical viewing location 3, at which a magnifying lens 6is located. FIG. 3 thus shows a conventional camera representing thestate of the art. At the large tilt-angle shown, the light-lobe 7exiting the focusing plane 2 is directed at such an angle to the opticalaxis 8 of the magnifying lens 6, that the brightness of the screen 2 isfairly low. In this application, i.e. in a large-format camera, themagnifying lens 6 may however be moved sideways so as to improve thescreen brightness, but the photographer is then forced to look throughthe eyepiece at an angle, which means that the magnifying lens 6 is usedin an optically very unfavourable manner.

By placing an optical wedge, here in the form of a fresnel prism plate 4shown in FIG. 4, the light-lobe exiting from the focusing screen 2 isredirected towards the magnifying lens 6 in its optimum positiondirectly above the optical axis of the focusing screen 2.

The invention is not restricted to the embodiments described above orshown in the drawings, but may be varied within the scope of theappended claims.

We claim:
 1. A device for facilitating the focusing of a camera during aso-called tilt operation, in which an image plane/focusing plane of thecamera is angled relative the optical axis of an objective lens (1) orvice versa, in order to obtain an inclined plane of focus, the camerabeing provided with a focusing screen (2) located in said imageplane/focusing plane and an optical viewing location (3), the opticalaxis of which coincides with the optical axis of said focusing screen(2), said tilt operation resulting in the exiting light from thefocusing screen being directed away from said viewing location (3),characterized in that optical wedge (4) is located between said focusingscreen (2) and said viewing location (3) in such a way as to redirectthe exiting light to said viewing location (3) at which an eyepiece (5)or an observer's eye is located.
 2. A device according to claim 1,characterized in that said optical wedge (4) is a Fresnel prism platelocated adjacent to said focusing screen (2) on the side of the exitinglight.
 3. A device according to claim 2, characterized in that aplurality of Fresnel prism plates (4), having progressively largerdeflection angles, are removably insertable adjacent to said focusingscreen (2).
 4. A device according to claim 3, characterized in that saiddeflection angles range from 0° to 30°.
 5. Method for facilitating thefocusing of a camera during a so-called tilt operation in which an imageplane/focusing plane of the camera is angled relative the optical axisof an objective lens (1) or vice versa, in order to obtain an inclinedplane of focus, the camera being provided with a focusing screen (2)located in said image plane/focusing plane and an optical viewinglocation (3), the optical axis of which coincides with the optical axisof said focusing screen, said tilt operation resulting in the exitinglight from the focusing screen being directed away from said viewinglocation (3), characterized in that an optical wedge (4) is placedbetween said focusing screen (2) and said viewing location (3) in such away as to redirect the exiting light to said viewing location (3) atwhich an eyepiece (5) or observer's eye is located.
 6. Method accordingto claim 5, characterized in that said optical wedge (4) is a Fresnelprism plate which is inserted adjacent to said focusing screen (2) onthe side of the exiting light.
 7. Method according to claim 6,characterized in that a plurality of Fresnel prism plates, havingprogressively larger deflection angles, are removably inserted adjacentto said focusing screen (2).
 8. Method according to claim 7,characterized in that said deflection angles range from 0° to 30°.